Neurons and glial cells



Neurons and glial cells



Nerve cells


The basic structure of the neuron has been outlined in Chapter 1. It was also shown how nerve cells can be classified based upon the type of information that they transmit (afferent, efferent, interneuron) or by the number of processes that they have (unipolar, bipolar, multipolar). Neuronal excitability, impulse conduction and synaptic transmission are discussed in Chapters 6 and 7.



Cortical neurons


The cerebral cortex contains more than 50 billion neurons arranged in horizontal layers or laminae. Although cortical neurons vary enormously in size and shape, there are two major types (discussed below).



Pyramidal and granule cells (Fig. 5.1A & B)


Pyramidal cells have large, pyramid-shaped cell bodies that range from 20–120 µm in diameter. They are excitatory neurons that have numerous apical and basal dendrites and a single axon that projects out of the cortex. Pyramidal cells are particularly prominent in motor and premotor areas. Granule cells (or stellate cells) are star-shaped multipolar neurons that have short axons and make local synaptic contacts, tending to be enriched in sensory cortices. They are much smaller than pyramidal cells, with a typical diameter of less than 20 µm, and may be excitatory or inhibitory. The cerebellar cortex also contains two main types of nerve cell: granule cells (similar to those in the cerebral cortex) and Purkinje cells (large efferent neurons, equivalent to cortical pyramidal cells; see Fig. 5.1C).




Cortical lamination


More than 90% of the cerebral cortex has a characteristic six-layered structure that appeared with the evolution of the mammalian brain (Fig. 5.2). For this reason it is referred to as neocortex (Greek: neos, new). Although the same six layers can be identified in all neocortical regions at some stage of development, they are not always present in the mature brain. For instance, the motor and premotor areas of the frontal neocortex are referred to as agranular cortex since they have lost their internal granule cell layer.




Different types of cortex


The cerebral cortex can be divided into more than fifty Brodmann areas based on subtle differences in the cortical structure (referred to as cytoarchitectonics) but there are three major cortical types (Fig. 5.3):




The majority of the ‘non-neocortical’ regions belong to the limbic lobe and are primarily concerned with emotion, memory and olfaction (the sense of smell). The term paralimbic cortex is used to describe non-neocortical regions outside of the limbic lobe proper, including the posterior orbital cortex, anterior insula and temporal pole.




Features of the neuron


Nerve cells have many features in common with other cells, together with a number of unique structural and functional components.





Subcellular organelles


The neuronal cell body or soma (Greek: soma, body) contains the same organelles found in other cell types (Fig. 5.5) but the machinery for protein synthesis and gene transcription is particularly prominent (Fig. 5.6A). The perinuclear cytoplasm or perikaryon (Greek: peri, around; cyton, kernel) contains a well-developed network of rough endoplasmic reticulum, often arranged in clumps called Nissl bodies. The Golgi apparatus is also prominent and is the site of post-translational modification and sorting of proteins including ion channels, neurotransmitter receptors and membrane ion pumps.







The neuronal cytoskeleton


All cells have a cytoskeleton composed of an internal framework of fibrillar proteins, that gives each cell its characteristic shape. This molecular scaffold is particularly important in process-bearing cells such as neurons and glia, which have a complex structure. The cytoskeleton is also involved in the transport of materials between intracellular compartments (see below). The main components include microtubules, neurofilaments and microfilaments (Fig. 5.7).







Axonal transport


The biological machinery for protein synthesis (in the neuronal cell body) may be quite a distance from the axon terminal. For this reason, the neuron has a fast axonal transport mechanism for membrane-bound materials and organelles:


Jun 19, 2016 | Posted by in NEUROLOGY | Comments Off on Neurons and glial cells
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